Intrinsic Viscosity

property

A measure of a polymer's molecular size and chain length in solution, expressed as the limiting viscosity number at infinite dilution, typically reported in dL/g or mL/g.

In Simple Terms

Think of intrinsic viscosity as a fingerprint for polymer chain length. Longer polymer chains create higher intrinsic viscosity values, while shorter or degraded chains show lower values. It's measured by dissolving the polymer in a specific solvent and testing how it flows.

Why It Matters

Intrinsic viscosity directly correlates with molecular weight and helps predict final product performance. For resin traders, it's a quality specification that determines pricing and suitability for specific applications like bottles versus films.

Technical Details

Measured using dilute solution viscometry, intrinsic viscosity ([η]) is calculated by extrapolating specific viscosity/concentration to zero concentration. The Mark-Houwink equation relates it to molecular weight: [η] = K × M^a, where K and a are polymer-solvent specific constants. Typical ranges: PET bottle grade 0.70-0.85 dL/g, HDPE 1.0-3.0 dL/g.

Real-World Examples

PET Bottle Grade Verification

A resin trader receives PET with claimed IV of 0.78 dL/g for bottle applications. Lab testing confirms 0.76 dL/g, slightly below spec but still acceptable for thin-wall bottles, affecting negotiated pricing.

HDPE Pipe Grade Selection

High-density polyethylene for pressure pipe requires IV above 2.5 dL/g to ensure adequate molecular weight for stress crack resistance and long-term durability in municipal water systems.

Recycled Content Quality Control

Post-consumer PET shows IV of 0.65 dL/g versus virgin at 0.82 dL/g, indicating chain scission during recycling. This lower IV limits use to fiber applications rather than food-grade bottles.